A packet radio system refers to a radio system which utilizes packet-switched technology known from fixed networks. Packet switching is a method of establishing a connection between the users by transmitting data in packets, which include address and control information. Several connections may simultaneously use the same transmission connection. Studies have concentrated on the use of packet-switched radio systems, in particular, because the packet switching method is well suited for data transmission where the data to be transmitted is generated in bursts. Thus it is not necessary to reserve the data transmission connection for the whole duration of transmission, but only for transmission of packets. This allows to save costs and capacity considerably both when the network is being constructed and used. When developing the GSM system further (Global System for Mobile Communication), researchers are paying special attention to packet networks, i.e. GPRS (General Packet Radio Service) and a radio system utilizing packet transmission (EGPRS, Enhanced General Packet Radio Service).
One of the main concerns in the design of prior art radio systems has been to guarantee the signal quality also when the channel quality is poor. In the planning of data transmission systems one essential parameter is the modulation method to be used on the transmission path. The information symbols to be transmitted cannot be transmitted as such over the transmission path because of the capacity of the transmission path and losses on it. Instead, the symbols have to be modulated by a suitable method to achieve a satisfying capacity of the transmission path and quality of transmission. In the development of prior art systems and selection of the modulation method the main concern has thus been to guarantee transmission quality, in which case the capacity of the modulation methods in poor channel conditions is important. As a result of this the ability of existing methods to transmit signals having a high data rate is rather poor. Thus it has been necessary to compromise the capacity in order to guarantee the quality of transmission.
An example of prior art modulation methods is GMSK (Gaussian Minimum Shift Keying), which is used in the GSM cellular radio system. It has a narrow frequency spectrum and good capacity, but the data transmission rates are not very high.
One solution to optimization of capacity and transmission rate is to change the modulation method to be used according to the need. When good interference tolerance is required, we can use the GMSK method, for example, and when the channel quality is good, we can employ the 8-PSK method, for example, which allows to achieve a data rate three times higher than that achieved with the GMSK.
The problem related to prior art radio systems is how to change the modulation method smoothly during a continuous connection. Change of the modulation method may cause problems in the receiver, in particular, because the transmitter may change the modulation method without informing the receiver of this in advance. Smooth change of the modulation method is, however, required in transmission of packet-switched data, for instance.
It is known from packet radio systems to transmit blocks consisting of bursts, e.g. four bursts. Packet radio systems employ various burst modulation methods and require a method for identifying the modulation method used on the downlink. Modulation remains the same during one four-burst block. For example, if modulation of one burst is interpreted incorrectly, a fourth of the information included in the block is lost. At most coding rates this prevents reception of the block.
Several subscriber terminals may use the same time slot. A base station transmits an uplink state flag on the downlink. State flags show which subscriber terminal is allowed to transmit signals on the uplink. The time slot can be utilized only if only one subscriber terminal uses it for data transmission. To realize this all subscriber terminals of the time slot should be able to receive uplink state flags without errors. The required detection ability is not dependent on modulation or the data coding rate.
In the GPRS system, for example, the uncoded uplink state flag consists of three bits per each four-burst radio block. An uncoded uplink state flag USF is coded into 6 or 12 bits depending on the data coding ratio, and the bits are interleaved into the data.